In recent years, imaging apparatuses having a plurality of imaging elements for taking an image of an object and an image-taking optical system for focusing light from the object on image-taking means have been developed widely for uses of e.g. X-ray CT apparatuses, duplicators, facsimile machines, scanners and camera modules including digital cameras, surveillance cameras and on-vehicle cameras.
In such imaging apparatuses, imaging elements are used to detect the intensity of light from an object, whereby an image is captured. However, it has been known that the amount of received light of respective imaging elements disagrees with actual signals owing to the drop in amount of light around a lens and the variations in sensitivities of imaging elements, causing an image nonuniformity in an image output. Hence, to eliminate such image nonuniformity, shading correction is performed.
For example, Japanese Unexamined Patent Publication JP-A 10-97617 (1998) discloses a method of performing data correction at the time of taking an image of an object based on shading correction data which have been prepared in advance based on white-reference-image-taking signals gained by taking an image of a white reference plate and black-reference-image-taking signals obtained by taking an image of a black reference plate. In addition, Japanese Unexamined Patent Publication JP-A 11-69154 (1999) discloses a correcting method of performing shading correction by use of histogram peak frequency values resulting from histogram peak frequency calculation performed for each of the imaging elements.
Further, Japanese Unexamined Patent Publication JP-A 2000-358142 discloses a correcting method which uses data gained from light coming out of a light source through a correcting member as shading data in parallel with irradiating an object with light from a light source. In addition, Japanese Unexamined Patent Publication JP-A 2005-80062 discloses a digital camera which performs shading correction by multiplying a shading correction factor in a first direction by a shading correction factor in a second direction perpendicular to the first direction.
It is disclosed in Japanese Unexamined Patent Publication JP-A 2005-269339 that first and second photosensitive sections differing in sensitivity are provided to perform color shading correction (correction of chromatic aberration) on each of imaging elements based on the difference in the amounts of light received by the two photosensitive sections, whereby color shading correction on each of pixels (imaging element) having a wide dynamic range can be conducted with a small memory capacity for a short time.
However, the correcting methods as disclosed by JP-A 10-97617, JP-A 11-69154, JP-A 2000-358142 and JP-A 2005-80062, by which shading data are prepared separately to conduct shading correction, have a problem that shading correction cannot be performed completely by a capturing way that light signals received by the imaging elements are read in one by one after the imaging elements have received signals from an object in parallel.
Specifically, causes of image nonuniformity include the fluctuation in amount of electric charge at the time of capturing signals into the imaging elements in addition to the drop in amount of light around a lens and the variation in sensitivity of imaging elements. For example, a method that images corresponding to one screen are captured in simultaneously like a global shutter technique using a CMOS sensor (Complementary Metal Oxide semiconductor sensor) is one which includes starting accumulation of electric charge accumulation for a plurality of imaging elements with the same timing, thereafter terminating the accumulation of electric charge with the same timing, and concurrently with the termination, taking out the electric charges accumulated in the imaging elements, which has an advantage that no image is distorted even when a target is moving rapidly because exposure is performed for all the taken images simultaneously. In such method, the time between termination of accumulation of electric charge and takeout thereof varies according to the respective imaging elements. On the other hand, the amount of electric charge accumulated in the imaging elements is increased owing to intrusion of electric charge brought about by various causes. However, it is known that the increase in the amount of electric charge varies depending on the amount of received light and the accumulation time. Hence, there is a problem that the image nonuniformity takes place because the amount of electric charge accumulated in the imaging elements increases before takeout thereof, and the amount of increased electric charge differs according to the imaging elements.
This is a result of the fact that the amount of electric charge accumulated in a portion lower a photosensitive section, such as a photodiode constituting an imaging element, intrudes into an electric charge accumulation section included in the imaging element before takeout of the amount of electric charge accumulated in the imaging element. Further, the amount of electric charge so intruding varies depending on the amount of electric charge accumulated in the imaging element. Therefore, image nonuniformity has been unable to be compensated completely by a conventional method such that shading data are prepared separately.
The fluctuation in the amount of electric charge to be corrected according to the amount of electric charge accumulated in the imaging element has been a phenomenon which takes place not only in the case of the global shutter technique using a CMOS sensor but also commonly in an imaging apparatus that images corresponding to one screen are captured into a plurality of imaging elements in parallel. Further, in any cases, an accurate shading correction method has been required.
In addition, according to the method including determining a difference between the amounts of light received by two photosensitive sections, and performing color shading correction on each of pixels based on the difference therebetween, as in JP-A 2005-269339 (2005), color shading correction on each of pixels can be performed, however, it has been impossible to compensate image nonuniformity resulting from the influence of leak of electric charge accumulated in each of imaging elements.